In the late 1960s anti-Vietnam war demonstrators tried to levitate the Pentagon by chanting mantras at it. Not surprisingly, the bastion of America's defense establishment stuck stubbornly to the ground. If the protestors had done a bit of research on the building's structure, however, they would have found they had a better chance of sinking it.

The Pentagon was completed on the Virginia shore of the Potomac in 1943. It is the world's largest office block, housing more than 25,000 workers in 3.7 million square feet of office space. It is also sinking into the ground, thanks to a fault in the foundations. Before building began, the site was covered with hydraulic fill -- dredged mud that can be easily pumped to provide a flat surface. But the fill was wet and poorly compacted. That is where the trouble started.

The building is supported on piles that are driven deep into the subsoil. But the concrete floor lies like a slab on the hydraulic fill. Over the years, the floor's weight has pressed the water out of the fill as if it were a sponge. Result: the fill has settled, leaving voids immediatley under the floor. If nothing is done, the floor cracks and subsides, something that has apparently happened already in parts of the building.

The solution sounds simple. Bore holes in the basement where the voids are and pump a mixture of concrete and bentonite slurry into them.

For a while this "mudjacking" (as it is known in the trade) works. The problem is that the slurry is an additional weight on the hydraulic fill that causes more water to filter out and more voids to appear. So more slurry has to be added, eventually causing more voids.

A discreet silence has been maintained in the Pentagon. Most employes know nothing about it, including those in the press office. "Oh really," said one press officer when The Economist told him what was happening. "I haven't felt the building move," said another more defensively. But a third did wonder what all the hoses were doing in the basement.

The cost of the operation is another mystery. The last time slurry had to be pumped in was almost a year ago when a company that had been contracted to install computers in the basement discovered that the floor was in danger of giving way. It called in the services of Technical Grouting Services Inc. to do the mudjacking and the bill was probably between $300,000 and $500,000. The General Services Administration says more slurry might be needed under a reconstructed kitchen.

The whole affair has been kept quiet not because national security is at stake or because the Pentagon is about to disappear in its very own meltdown, but simply to avoid embarrassment. Officials did not want to cope with a battery of jokes at their expense. Now they may have to.